Recently, there has been an increased interest in tailored development of polyaromatic polymers, particularly polyaromatic polymers that are electrically conductive and/or have useful optical properties. Examples of electrically conductive polymers include certain polyanilines, polythiophenes, polypyrroles, and polyphenols. These conductive polyaromatic polymers may be used in a variety of electronic devices, including electro-chromic devices, light-emitting diodes, electrostatic discharge protection, and light weight batteries. Of these polyaromatic polymers, polyanilines are the most extensively studied, due largely to superior electrical properties, such as high discharge capacity.
In addition to the above-named electrical properties, thermal and structural properties of polyphenols have long been exploited. In particular, phenol-formaldehyde resins, such as novolacs and resols, have found wide application as wood composites, laminates, foundry resins, abrasives, friction and molding materials, coatings and adhesives, fiber binders, and flame retardants. The use of formaldehyde in polyphenol synthesis, however, presents a significant toxicological and environmental hazard.
Despite the industrial utility of polyaromatic polymers, their synthesis remains problematic. Known difficulties in the. synthesis of such polymers include inconsistent product composition, due in part to extensive branching of the polymers. In addition, many of the polyaromatic polymers are insoluble, or sparingly soluble, in common solvents, leading to poor processability. The use of toxic reagents, as noted above, is another undesirable feature of current synthetic methods. A search for new methods of synthesizing polyaromatic polymers has not heretofore yielded a commercially viable approach.
Many of the synthetic approaches to forming polyaromatic polymers use a heme-containing enzyme to catalyze the polymerization. Any such catalyst must necessarily be stable and active under acidic conditions, as acidic conditions are required in order to synthesize an electrically conductive form of a polyaromatic polymer, such as polyaniline.
The increasing environmental problem of hazardous chemical wastes has led to an upsurge in efforts toward the development of biochemcial alternatives for synthesis of electronic and photonic polymers. Enzymatic polymerizations have attracted much attention with oxidative enzymes, such as horseradish peroxidase (HRP), being used for the synthesis of polyanilines and polyphenols through oxidative free radical coupling reactions.
Unfortunately, HRP and other peroxidases are inactive at low pH and are prohibitively expensive to use commercially. Hematin has been used to mimic the catalytic activity of HRP. However, despite its lower costs, hematin is a non-ideal catalyst for commercial polymerizations because of its low solubility in acidic, aqueous media. The low solubility of hematin under these conditions leads to an extremely low rate of polymerization and very poor yields.
The mechanism for HRP catalyzed polymerization involves the interaction of the heme-iron cofactor of the enzyme with the peroxide yielding an oxidized heme-iron complex. Subsequently, the oxidized heme-iron complex reacts with the substrate in a one-electron transfer reaction to produce the substrate radical and a new iron-heme complex followed by the coupling of the radicals to form the polymer.
This enzymatic approach has not been extended to polythipophenes or polypyrroles, which have high electrical conductivity. This is because monomers, such as (3,4)-ethylenedioxythiophene (EDOT) and pyrrole (PYR), complexed with the active site of the enzyme catalyst cause deactivation of the latter and have proved to be unsuitable substrates for this enzymatic polymerization. This deactivation phenomenon drastically limits the prospects for the enzymatic synthesis of a wide range of polymers for possible industrial applications. The present invention evolved from exploration of the possibility of usage of a modified hydroxy ferriprotoporphyrin Hematin to serve as a catalytic center.
There is a need for a low cost, high efficiency means of synthesizing polyaromatic electronic and photonic polymers, which means is compatible with conditions required to synthesize polymers with commercially desirable properties.